1. Field of the Invention
This invention relates to a display apparatus, a driving method for a display apparatus and an electronic apparatus, and more particularly to a display apparatus of the flat type or flat panel type wherein a plurality of pixels are arranged two-dimensionally in a matrix, a driving method for the display apparatus and an electronic apparatus which incorporates the display apparatus.
2. Description of the Related Art
In recent years, in the field of display apparatus which display an image, a flat type display apparatus wherein a plurality of pixels or pixel circuits are arranged in a matrix, that is, in rows and columns, has been popularized rapidly. One of such flat type display apparatus uses, as a light emitting element of a pixel, an electro-optical element of the current driven type whose emitted light luminance varies in response to the value of current flowing through the element. As the electro-optical element of the current driven type, an organic EL (Electro Luminescence) element which utilizes a phenomenon that an organic thin film emits light when an electric field is applied thereto is known.
An organic EL display apparatus which uses an organic EL element as an electro-optical element of a pixel has the following characteristics. In particular, the organic EL element has a low-power consumption characteristic because it can be driven by an application voltage equal to or lower than 10 V. Since the organic EL element is a self luminous element, it displays an image of high visibility in comparison with a liquid crystal display apparatus which displays an image by controlling the intensity of light from a light source using liquid crystal for each pixel. Besides since the organic EL element does not require an illuminating member such as a backlight, it facilitates reduction in weight and thickness of the organic EL display apparatus. Further, since the speed of response is as high as approximately several μsec, an after-image upon dynamic picture display does not appear.
The organic EL display apparatus can adopt a simple or passive matrix type or an active matrix type as a driving method therefor similarly to the liquid crystal display apparatus. However, although the display apparatus of the simple matrix type is simple in structure, it has a problem in that it is difficult to implement the same as a large-sized high definition display apparatus because the light emitting period of each electro-optical element decreases as the number of scanning lines, that is, the number of pixels, increases.
Therefore, in recent years, development of an active matrix display apparatus wherein the current to flow through an electro-optical element is controlled by an active element provided in a pixel in which the electro-optical element is provided such as an insulated gate type field effect transistor has been and is being carried out vigorously. As the insulated gate type field effect transistor, usually a thin film transistor (TFT) is used. The active matrix display apparatus can be easily implemented as a large-sized and high definition display apparatus because the electro-optical element continues to emit light over a period of one frame.
Incidentally, it is generally known that the I-V characteristic, that is, the current-voltage characteristic, of the organic EL element deteriorates as time passes (aged deterioration). In a pixel circuit which uses a TFT particularly of the N channel type as a transistor (hereinafter referred to as driving transistor) for driving the organic EL element by current, if the I-V characteristic of the organic EL element suffers from aged deterioration, then the gate-source voltage Vgs of the driving transistor varies. As a result, the luminance of emitted light of the organic EL element varies. This arises from the fact that the organic EL element is connected to the source electrode side of the driving transistor.
This is described more particularly. The source potential of the driving transistor depends upon the operating point of the driving transistor and the organic EL element. Then, if the I-V characteristic of the organic EL element deteriorates, then since the operating point of the driving transistor and the organic EL element varies, even if the same voltage is applied to the gate electrode of the driving transistor, the source potential of the driving transistor changes. Consequently, the source-gate voltage Vgs of the driving transistor varies and the value of current flowing to the driving transistor changes. As a result, since also the value of current flowing to the organic EL element varies, the emitted light luminance of the organic EL element varies.
Further, particularly in a pixel circuit which uses a polycrystalline silicon TFT, in addition to the aged deterioration of the I-V characteristic of the organic EL element, a transistor characteristic of the driving transistor varies as time passes or a transistor characteristic differs among different pixels due to a dispersion in the fabrication process. In other words, a transistor characteristic of the driving transistor disperses among individual pixels. The transistor characteristic may be a threshold voltage Vth of the driving transistor, the mobility μ of a semiconductor thin film which forms the channel of the driving transistor (such mobility μ is hereinafter referred to simply as “mobility μ of the driving transistor”) or some other characteristic.
Where a transistor characteristic of the driving transistor differs among different pixels, since this gives rise to a dispersion of the value of current flowing to the driving transistor among the pixels, even if the same voltage is applied to the gate electrode of the driving transistor among the pixels, a dispersion appears in the emitted light luminance of the organic EL element among the pixels. As a result, the uniformity of the screen image is damaged.
Therefore, various correction or compensation functions are provided to a pixel circuit in order to keep the emitted light luminance of the organic EL element fixed without being influenced by aged deterioration of the I-V characteristic of the organic EL element or aged deterioration of a transistor characteristic of the driving transistor as disclosed, for example, in Japanese Patent Laid-Open No. 2006-133542.
The correction functions may include a compensation function for a characteristic variation of the organic EL element, a correction function against the variation of the threshold voltage Vth of the driving transistor, a correction function against the variation of the mobility μ of the driving transistor and some other functions. In the description given below, the correction against the variation of the threshold voltage Vth of the driving transistor is referred to as “threshold value correction,” and the correction against the mobility μ of the driving transistor is referred to as “mobility correction.”
Where each pixel circuit is provided with various correction functions in this manner, the emitted light luminance of the organic EL element can be kept fixed without being influenced by aged deterioration of the I-V characteristic of the organic EL element or aged deterioration of a transistor characteristic of the driving transistor. As a result, the display quality of the organic EL display apparatus can be improved.
The compensation function for a characteristic variation of the organic EL element is executed by such a series of circuit operations as described below. First, an image signal supplied through a signal line is written by a writing transistor so as to be stored into a storage capacitor connected between the gate and the source of the driving transistor. Thereafter, the writing transistor is placed into a non-conducting state to electrically disconnect the gate electrode of the driving transistor from the signal line to place the gage electrode of the driving transistor into a floating state.
When the gate electrode of the driving transistor is placed into a floating state, since the storage capacitor is connected between the gate and the source of the driving transistor, also the gate potential Vg of the driving transistor varies in an interlocking relationship with, that is, following up, the variation of the source potential Vs of the driving transistor. An operation for varying the gate potential Vg in an interlocking relationship with the source potential Vs of the driving transistor in this manner is hereinafter referred to as bootstrap operation. By this bootstrap operation, the gate-source voltage Vgs of the driving transistor can be kept fixed. As a result, even if the I-V characteristic of the organic EL element suffers from aged deterioration, the emitted light luminance of the organic EL element can be kept fixed.